This invention relates to the beneficiation of coal and similar carbonaceous solids which contain impurities in the form of ash-forming, inorganic constituents, commonly referred to as mineral matter, and inorganic and organic sulfur.
Known resources of coal and other solid carbonaceous fuel materials in the world are far greater than the known resources of petroleum and natural gas combined Despite this enormous abundance of coal and related solid carbonaceous materials, reliance on these resources, particularly coal, as primary sources of energy, has been discouraged for the most part. The availability of cheaper, cleaner burning, more easily retrievable and transportable fuels, such as petroleum and natural gas, has in the past, cast coal to a largely supporting role in the energy field.
As a result, enormous efforts are being extended to make coal and related solid carbonaceous materials equivalent or better sources of energy, than petroleum or natural gas. In the case of coal, for example, much of this effort is directed to overcoming the environmental problems associated with its production, transportation and combustion. For example, health and safety hazards associated with coal mining have been significantly reduced with the onset of new legislation governing coal mining. Furthermore, numerous techniques have been explored and developed to make coal cleaner burning, more suitable for burning and more readily transportable.
Gasification and liquefaction of coal are two such known techniques. Detailed descriptions of various coal gasification and liquefaction processes may be found, for example, in the Encyclopedia of Chemical Technology, Kirk-Othmer, Third Edition (1980) Volume 11, pages 410-422 and 449-473. However, these techniques, typically, require high energy input, as well as the utilization of high temperature and high pressure equipment, thereby reducing their widespread feasibility and value.
In addition to gasification and liquefaction, other methods for converting coal to more convenient forms for burning and transporting are also known. For example, the preparation of coal-oil and coal-aqueous mixtures are described in the literature. Such liquid coal mixtures offer considerable advantages. In addition to being more readily transportable than dry solid coal, they are more easily storable, and less subject to the risks of explosion by spontaneous ignition. Moreover, providing coal in a fluid form makes it feasible for burning in conventional apparatus used for burning fuel oil. Such a capability can greatly facilitate the transition from fuel oil to coal as a primary energy source.
Regardless of the form in which the coal is ultimately employed, the coal or coal combustion products must be cleaned because they contain substantial amounts of sulfur, nitrogen compounds and mineral matter, including significant quantities of metal impurities like, aluminosilicates, metal oxides, metal pyrites, metal sulfates, etc. During combustion these materials enter the environment as sulfur dioxide, nitrogen oxides and compounds of metal impurities. If coal is to be accepted as a primary energy source, it must be cleaned to prevent pollution of the environment either by cleaning the combustion products of the coal or cleaning the coal prior of burning.
Accordingly, physical as well as chemical coal cleaning (beneficiation) processes have been explored. In general, physical coal cleaning processes involve pulverizing the coal to release the impurities, wherein the fineness of the coal generally governs the degree to which the impurities are released. However, because the costs of preparing the coal rise exponentially with the amount of fines to be treated, there is an economic optimum in size reduction. Moreover, grinding coal even to extremely fine sizes may not be effective in removing all the impurities. Based on the physical properties that effect the separation of the coal from the impurities, physical coal cleaning methods are generally divided into four categories: gravity, flotation, magnetic and electrical.
In contrast to physical coal cleaning, chemical coal cleaning techniques are in a very early stage of development. Known chemical coal cleaning techniques include oxidative desulfurization of coal (sulfur is converted to a water-soluble form by air oxidation), ferric salt leaching (oxidation of pyritic sulfur with ferric sulfate), and hydrogen peroxide-sulfuric acid leaching. Other methods are also disclosed in the above-noted reference to the Encyclopedia of Chemical Technology, Volume 6, pages 314-322.
Furthermore, the patent literature is replete with chemical coal beneficiation processes. For example, U.S. Pat. No. 4,424,062 discloses a process for chemically removing ash from coal by immersing ash containing coal in an aqueous solution containing hydrochloric acid or citric acid in combination with acidic ammonium fluoride. U.S. Pat. No. 3,993,455 discloses a process for removing mineral matter from coal by the treatment of the coal with aqueous alkali such as sodium hydroxide, followed by acidification with strong acid. Similarly, U.S. Pat. No. 4,055,400 discloses a method of extracting sulfur and ash from coal by mixing the coal with an aqueous alkaline solution, such as ammonium carbonate.
U.S. Pat. No. 4,071,328 discloses a method of removing sulfur from coal by first hydrogenating the coal and the hydrogenated coal is subsequently contacted with an aqueous inorganic acid solution. U.S. Pat. No. 4,127,390 discloses a process for reducing the sulfur content of coal by treatment with an aqueous sodium chloride solution. U.S. Pat. No. 4,134,737 discloses a process for the production of beneficiated coal wherein the coal is digested in caustic, then treated in mineral acid and then treated in nitric acid.
U.S. Pat. No. 4,083,940 discloses a process for cleaning coal by contacting the coal with an aqueous leaching solution containing nitric and hydrofluoric acid. U.S. Pat. No. 4,169,710 discloses comminuting and cleaning coal of sulfur and ash by contacting the coal with a hydrogen halide, such as HF (aqueous and/or anhydrous).
U.S. Pat. No. 4,408,999 discloses beneficiating coal by subjecting the coal to electromagnetic radiation in the presence of a strong inorganic acid, such as hydrofluoric acid. In turn, U.S. Pat. No. 4,305,726 discloses a chemical method of treating coal to remove ash and sulfur comprising treating the coal with hydrochloric and hypochlorous acid in the presence of ferric and ferrous sulfate, while U.S. Pat. No. 4,328,002 discloses a method of treating coal to remove ash and sulfur involving preconditioning coal particles in the presence of an aqueous solution of an oxidant, such as H.sub.2 O.sub.2 or HF, washing the so-treated coal, treating the washed coal with further oxidant and then passivating the coal with for example, an ammonium salt and then neutralizing with alkali metal hydroxide.
U.S. Pat. No. 4,516,980 discloses a process for producing low-ash, low sulfur coal by a two-stage alkaline treatment using sodium carbonate or bicarbonate as the reagent. The alkaline treated coal is then extracted with aqueous mineral acid; and U.S. Pat. No. 3,998,604 discloses a coal demineralization process whereby ground coal is treated with aqueous acid, such as HCl, H.sub.2 SO.sub.4 or H.sub.2 CO.sub.3 and then subjected to froth flotation in the presence of a gas selected from Cl.sub.2, SO.sub.2 or CO.sub.2.
Although HCl has been found effective in the removal of certain types of mineral matter from coal, processes that utilize HCl in any form run the risk of chlorinating the aromatic and heteroatomic organic matrix found in coal. The chlorine cannot be removed from the chlorinated coals by simple washing or drying under vacuum. The corrosiveness of Cl liberated from combusted coal is well known. On the other hand, while it is also known that HF is very effective in removing silica and alumina from coal, it is not so effective in removing divalent alkali metals, such as calcium and magnesium. Furthermore, as also evidenced above, several prior art processes utilize oxidizing acids such as HNO.sub.3 and H.sub.2 SO.sub.4. Although they may aid in the removal of mineral matter, they are also very capable of oxidizing the organic coal matrix, thereby decreasing the amount of volatile matter and the heating value of the coal.